I think it stems from his argument being not about what the law says, but about if the law is constitutional.
Very often things that involve political figures are much more likely to get a very generous interpretation of the first amendment, which is why you get stuff like “elected officials can’t always block people on social media even with their personal accounts”.
They claimed that preventing him from trademarking the slogan limited his ability to monetize it, and that made it a limitation on his freedom of speech, specifically regarding a politician. Therefore the government should need to provide explicit, compelling reason for the law as applied to politicians. Recently, trademark rules were shot down over first amendment grounds when the supreme Court found that rules saying you can’t trademark insulting or vulgar things amounted to the government prohibiting speech in a way it’s not allowed to.
With this ruling, they found that the rule in question is viewpoint neutral and therefore isn’t the government disfavoring an idea or viewpoint. It’s unbiased since it’s based on (hopefully) objective facts about if people are alive or not, unlike “is FUCT a vulgar word” or “is it disparaging to name a band The Slants”?